Television pickup tube



G. A. MORTON ETAL TELEVISION PICKUP TUBE March 28, 1944.

Filed Aug. 29, 1941 .llll

lI'IIII'V Inventors yeflmi'ian 619' 62% 6L 23/ g Qttorneg Patented Mar. 28, 1944 UNITED STATES PATENT OFFICE TELEVISION PICKUP TUBE George A. Morton, Haddon Heights, and Leslie E. Flory, Oaklyn, N. J., assignors to Radio Corporation oi America, a corporation of Delaware Application August .29, 1941, Serial No. 408,882

6 Claims.

This invention relates to electron beam tubes and to methods of operating the same and has special reference to the provision of improvements in television pickup tubes of the type employing a scanning beam of low velocity.

The usual television pickup tube or "Iconoscope" comprises a target electrode constituted of a multiplicity of minute photosensitive particles supported in insulating relation upon a suitable back plate." It is upon this target or "mosaic" electrode that an optical image of the scene to be transmitted is impressed. In addition to the mosaic electrode such tubes include an electron source and means are provided whereby a beam of electrons from the source may be caused to "scan the photosensitive particles so that the electrical positive charges acquired by the said particles (by reason of the emission therefrom of photoelectrons) are sequentially neutralized and produce a train of electrical impulses in an output circuit which is usually coupled to the back plate of the mosaic. Optimum performance of such tubes is ensured when the amplitude of these electrical impulses bear a. predetermined relation to the intensity of the light received by these elemental areas upon the impress of the optical image thereon.

One very real difllculty encountered in the operationof television pickup tubes of the type wherein the scanning beam impinges upon the mosaic at a. very low velocity, is that when the mosaic is subjected to illumination of an intensity exceeding a. certain value the usual scanning beam is not able to bringit back to equilibrium and the potential on the mosaic will continue to go positive to a point where the scanning beam loses control altogether, in which case a useful picture signal is no longer obtained. If,

in an eilort to solve this difilculty, the beam current is increased, the maximum safe illumination before this condition sets in, of course, becomes greater. However, very large beam currents cannot-be obtained without loss of resolution.

Accordingly, the principal object of the present invention is to obviate the foregoing and other less apparent objections to tubes employing low velocity scanning.

Another and important object of the present invention is to provide an improved apparatus for converting a light image into corresponding electrical signals, and one incorporating means for controlling the "gamma of the image reproduced from said signals.

In accordance with the invention a television 55 pickup tube utilizing. a scanning beam of low velocity is provided with an auxiliary cathode which, considered from one aspect, creates a reservoir from which electrons may be drawn to themosaic whenever the intensity of the light thereon is such as to give rise to a predetermined crltical positive charge. The auxiliary cathode is normally energized but is maintained at a potentiabsuch that in the absence of an excessive positive charge upon the mosaic none of the electrons from'the said source impinges upon the mosaic, but are collected elsewhere in the tube. Thepotential or bias upon the auxiliary cathode is preferably adjustable and hence, in addition to preventing the mosaic from becoming too positive, may be used for controlling the contrast or gamma of the image which is eventually reproduced from the electrical signals derived from the optical or light image upon the for, constructed and arranged in accordance with the principle of the invention, and

Figure 2 is a similar view of another embodiment of th invention, and

Figure 3 is a sectional view taken on the line 3-3 of Fig. 2.

The television transmitting tube shown in Fig. 1 comprises a highly evacuated envelope T which has a long axis :c-a: and contains, adjacent to one end, an electron gun indicated generally at G, and, adjacent to its opposite end, a conventional transparent mosaic electrode M comprising a multiplicity of mutually insulated light-sensitive elements 2 which, in this case, are supported on the inner surface of the leading end of the tube. The electron gun G is preferably, though not necessarily, mounted in register with the long axis x-a: of the tube and in this case comprises an indirectly heated cathode 4, a control electrode 6 and one or more accelerating electrodes 8.

An auxiliary source of electrons, indicated generally at V, is shown mounted parallel to the gun G oil the central axis :c-a: in the space between the gun and the inner surface of the tube. This auxiliary electron source V may be of a construction'similar to that of the gun G and is shown as comprising an indirectly heated cathode H], a control electrode l2 and one or more accelerating electrodes I4. A preferably disc-like electrode l6 serves as a common focusing electrode for the electrons from the separate sources G and V and to this end is provided with apertures g and 1), respectively, through which the electrons pass in their journey to or toward the mosaic. Other electrodes in the tube are: a second anode l8 and two decelerating rings 20 and 22, respectively. These last mentioned electrodes i8, 20 and 22 may conveniently comprise film-like metallic deposits applied directly to the inner surface of the envelope T. Deflecting devices, such as defiecting coils 2d and 26, are provided for deflecting both electron beams in a horizontal plane and in a vertical plane.

In operating the device of Fig. 1, an optical image of the object or scene to be televised is impressed (as by means of a suitable lens system, not shown) upon the mosaic M through the end wall of the glass envelope T. A5 is well known in the art, the elemental areas 2 of the mosaic M thus each acquire an electrostatic potential proportional to the intensity of the light which impinges thereon. The particles or elements 2 of the mosaic which are the more highly illuminated acquire a more positive charge than the less illuminated or unilluminated particles. Each particle retains its charge until discharged through a resistor R in the output or signal transmitting circuit.

If the light which is directed upon the mosaic M is of average or normal intensity the positive electrostatic charge which is developed on any of its elemental areas 2 will be efiectively neutralized or discharged by electrons from the gun G when the beam from the said gun is moved across the mosaic under control of the deflecting coils 24 and 25. It will be observed that the focusing electrode l6 and the second anode i8 are maintained at a relatively high potential (say 1000 volts positive with respect to the cathode 4 of the gun G) and that the decelerating electrodes 20 and 22, and the mosaic electrode M are connected to points of successively lower potentials hence, as in the usual low velocity scanning system, the electrons from the gun G approach the mosaic at a very low or zero velocity. As is usual in such a case, when the electrons of the scanning beam are directed to an element of the mosaic which is negative with respect to the cathode 4 of the gun G, they cannot reach the said element because of their low velocity but are drawn in the return direction and collected either upon the second anode It or the focusing electrode is.

The auxiliary source of electrons V with which the device of the invention is provided is normally energized but its cathode l and grid l2 are maintained slightly (from a fraction of a volt to say twenty volts) positive with respect to the corresponding electrodes 4 and 6 of the gun G. Thus, when the charge uponany of the elemental areas 2of the mosaic M is below a selected critical voltage, electrons from the said auxiliary source V will not reach the mosaic M but will comprise a virtual source" or "reservoir" of electrons whichllike the non-impinging of the electrons from the gun G) are eventually collected upon one or both of the electrodes l6 and.

I8. On the other hand, if, because of excessive illumination or other cause, the mosaic M tends to go positive above a safe maximum the beam from the auxiliary cathode V strikes it and prevents a further rise in potential. As soon as the excessive illumination or other cause is removed the mosaic returns to its normal equilibrium instead of staying positive, as it would if electrons from the auxiliary source V were not available for purposes of neutralizing the saidexcessive charge.

Where, as in the above described embodiment of the invention, the electrons from the auxiliary source V are in the form of a beam when drawn to the mosaic M, a second picture signal of relatively low resolution will be generated. This second picture signal is usually not noticeable, since it coincides more or less with the signal generated by the beam from the gun G and in any event is usually not of appreciable duration since the potential of the source V is such that it can afiect the mosaic only during the emergency. I

In the embodiment of the invention shown in Figs. 2 and 3, the auxiliary source of electrons instead 01' comprising an electron gun (V, Fig. 1) comprises an electron-emissive ring-like element V which surrounds the axis of the tube in front of the focusing electrode l5 through which passes the scanning beam from the gun G. This gun G is mounted off the central axis of the tube and the beam therefrom is caused to scan the mosaic by paired horizontal and vertical deflecting coils 24, 26, 28, 30, respectively, surrounding the tube. An outer coil 32 supplies the axial magnetic field necessary to maintain the focus of the beam from the gun G both in the direction of the mosaic and in the return direction.

The mosaic electrode M is here shown connected to ground and the output circuit is coupled to a collector electrode C adjacent to the gun end of the tube, behind an aperture 0 in the focusing electrode IS. The electrons which are collected by output electrode C comprise those low velocity electrons from the gun G which fail to reach the mosaic electrode. Since some of the electrons in the original beam are drawn to the mosaic by the positive charges, thereon (caused by the impress of the optical image), the beam which actually reaches the collector electrode C will be modulated in accordance with the pattern of the charge upon the mosaic. A pair of lifting electrodes 33 and 34, respectively, which, as shown more clearly in Fig. 3 may comprise a split ring or cylinder on the inner wall of the tube in front of the focusing electrode l6, serve in known manner to direct the nonimplnging electrons from the gun G to collector C.

Only the electrons from the gun G can enter the aperture 0. The electrons from the auxiliary electrode V which fail to reach the mosaic M are collected elsewhere in the tube and thus in no wise affect the current to the collector C. If desired, the collector electrode may comprise the first stage of an electron multiplier device.

When the auxiliary cathode is used for controlling the'gamma of the image, means are preferably provided for selecting between the electrons from the auxiliary cathode and those from the scanning gun. This differentiation occurs in the case of the multiplier iconoscope (see Fig. 2)

because the electrons from the spray beamdo tor frequency. This signal is then rectified,

which converts it into a normal video signal. Where the electrons from the auxiliary cathode are not made to scan the mosaic the diilerentiation is not necessary, as only the electrons in the scanning beam can give rise to a varying current v in the signal load Other embodiments and modifications oi the invention will suggest themselves to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative 3. The invention asset forth in claim 2 and wherein the electrons from said auxiliary source are drawn to said mosaic in the form 01' a beam.

and not in a limiting sense exceptas required bythe spirit of the appended claims.

What is claimed is: I

1. Method of operating an electron tube having a mosaic electrode adapted to permit the impress of an optical image thereon, said method comprising scanning said mosaic with a beam of low velocity electrons, establishing an auxiliary source of electrons in said tube, and drawing electrons from said auxiliary source to said mosaic only when the intensity of the rays comprising said optical image exceeds a predetermined value.

' 2. Method of operating an electron tube having a mosaic electrode adapted. to permit the impress of an optical image thereon, said method comprising scanning said mosaic with a beam of low velocity electrons, establishing an auxiliary source of electrons in said tube, and draw-v ing electrons from said auxiliary source to said mosaic only when the electrical charge on said mosaic incident to the impress thereon oi said optical image exceeds a predetermined positive value. 7

4. The invention as set iorth in claim 2 and wherein the electrons from said auxiliary source are drawn to said mosaic in the form 01 a spray.

5. Method of operating an image tube having a mosaic electrode ,comprising a multiplicity of light sensitive elements, said method comprising scanning said mosaic with an electron beam having a current density calculated to restore said light sensitive elements to an equilibrium potential when said mosaic is subject to light of normal intensity, establishing an auxiliary source of electrons in said tube, and then directing electrons for drawing electrons from said auxiliary source to said mosaic electrode.

GEORGE A. MORTON. asrm E. near. 

